The use of sintered powder metal (PM) parts has accelerated in the recent past for components difficult to manufacture by other methods as PM components can offer a cost effective alternative to other metal formed components. Some advantages of powder metallurgy include lower costs, improved quality, increased productivity and greater design flexibility. These advantages are achieved in part because PM parts can be manufactured to net-shape or near-net shape which yields little material waste, and which in turn eliminates or minimizes machining. Other advantages of the PM manufacturing process and parts produced therefrom, particularly over other metal forming processes, include greater material flexibility including graded structures or composite metal, lighter weight of the parts, greater mechanical flexibility, reducing energy consumption and material waste in the manufacturing process, high dimensional accuracy of the part, good surface finish of the part, controlled porosity for self-lubrication or infiltration, increased strength and corrosion resistance of the component, and low emissions, among others.
Internal combustion engine manufacturers have sought more efficient, cost effective and viable ways to reduce cost and weight in engines without sacrificing performance and/or safety. One of the largest and most important components of the engine is the cylinder block. In the past, cylinder blocks had been formed from cast iron, which provided strength, durability and long service life. However, as can be appreciated, cast iron is quite heavy. Further, cast iron has a relatively poor thermal conductivity. Consequently, alternatives to cast iron cylinder blocks are sought.
One such alternative is to form the blocks from aluminum. Aluminum is very lightweight and has good thermal conductivity, each of which are desirable features in the engine industry. However, aluminum is relatively soft and easily scratched and thus does not provide the strength, durability and long service life required for use in a cylinder block, particularly with respect to the requirements of the cylinder bores in the block. Further, aluminum has a relatively high coefficient of thermal expansion compared to iron, which can increase blowby between a cylinder and piston during combustion at high operating temperatures, thereby increasing emissions.
As an alternative, engine manufacturers have used more wear resistant cylinder liners within the cylinder bores of an aluminum block. Cylinder liners are typically in-cast into aluminum engine blocks to provide improved wear resistance compared to the aluminum bore that is present without the liner. A cast iron, machined cylinder liner is typically used for engines that require a cylinder liner. However, these cast iron cylinder liners have a less than desirable mechanical bond with the aluminum engine block which leads to less than desirable heat transfer properties. Further, features are required on the outside of the cast iron cylinder liner to “lock” in place in the aluminum block, and these features can create an uneven heat transfer from the cast iron cylinder liner to the aluminum block, or undesirable voids or local hot spots can be created between the liner and the aluminum. Additionally, the alloys used in cast iron cylinder liners are not optimum relative to strength and stiffness, resulting in bore distortion during combustion, more blow-by and higher emissions.
The inherent porosity of a powder metal iron alloy part, when in-cast into an aluminum casting, allows the molten aluminum to infiltrate the matrix of the PM part to improve the bond between the surrounding aluminum and the PM part. Allowing penetration of the molten aluminum into the cylinder liner porosity also takes advantage of the desirable machinability of the impregnated PM matrix. Further, the alloys which can be used for a PM part allow for higher strength and stiffness when compared to a cast iron part.
Although PM technology has the potential of overcoming some of the problems with cast iron cylinder liners, production of PM cylinder liners by conventional axial compaction to net shape or near net shape has not been commercially feasible. One reason is that the high length to wall thickness ratio results in excessive difficulties filling the compaction die with metal powder. In addition, compacting from the ends of a part with a high aspect ratio results in an unacceptable density gradient along the length of the cylinder liner, and inadequate green strength of the compact. These problems can be somewhat overcome using cold isostatic compaction plus subsequent secondary manufacturing operations, but can be too costly in comparison with cast cylinder liners.
While the above discussion has been directed to cylinder liners, other devices having a high length to wall thickness ratio, such as bushings, and electric motor stators or armatures for example, have similar problems when attempting to produce these parts using powder metal technology.